1. Field of the Invention
The present invention relates to a power supply circuit, and particularly relates to a power supply circuit that generates a desired output voltage by voltage multiplication.
2. Description of the Related Art
Within a semiconductor integrated circuit, there has been widely used a voltage step-up/down circuit that performs voltage multiplication of a power supply voltage to generate a desired voltage. For example, in a liquid crystal driver that drives a LCD (Liquid Crystal Display) panel, the voltage step-up/down circuit performs voltage multiplication of an internal reference power supply voltage Vci to thereby generate a drive voltage of the liquid crystal display panel. To be specific, Japanese Patent Application Publication No. 2004-4609 discloses a technique in which a capacitor and a switch quadruple an internal reference power supply voltage Vci to generate a voltage V0; and then the voltage V0 is voltage-divided to thereby generate voltages V1 to V4. Moreover, International Patent Publication Application WO2002/061931 discloses a boost type power supply circuit including: a differential amplifier that generates a reference voltage Va; and a charge pump that generates an output voltage Vout having a voltage level of Va+2 VDD from the reference voltage Va and a power supply voltage VDD. The reference voltage Va is controlled by feeding back the output voltage Vout. More specifically, the differential amplifier compares a voltage obtained by voltage-dividing the output voltage Vout with a reference voltage Vref to generate the reference voltage Va.
However, such known voltage step-up/down circuits cannot satisfy both a demand to reduce current consumption and a demand to enable operation with a low power voltage at the same time. For example, consider a case of a liquid crystal driver. Assuming that a drive voltage of the liquid crystal display panel is Vo and that a drive current is Io, the power consumption in the liquid crystal display panel is generally expressed as Vo×Io. Therefore, if the drive voltage Vo is obtained by tripling an internal reference power supply voltage Vci, a current three times as large as the drive current Io (that is, 3Io) is consumed in an internal power supply circuit that generates the internal reference power supply voltage Vci.
If the multiplication rate for voltage multiplication is reduced, it is possible to reduce power consumption in the internal power supply circuit that generates the internal reference power supply voltage Vci. For example, when the drive voltage Vo is generated by doubling the internal reference power supply voltage Vci, a current consumed in the internal power supply circuit that generates the internal reference power supply voltage Vci is reduced to a current only twice as large as the drive current Io (that is, 2Io). However, in this case, the internal reference power supply voltage Vci must be increased in order to generate the necessary drive voltage Vo.
The aforementioned problem is generally applied to any circuits, which obtain a desired voltage by voltage multiplication, other than that for the liquid crystal driver.